1. Field of the Invention
The present invention relates to calibration technologies and it particularly relates to a calibration method and a radio apparatus utilizing the same in adaptive array antennas.
2. Description of the Related Art
In wireless communication, it is generally desired that the limited frequency resources be used effectively. Adaptive array antenna technology is one of the technologies that can realize effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. In other words, the apparatuses provided with adaptive array antennas change the amplitude and phase of signals received by a plurality of antennas, respectively, add up the thus changed received signals, and receive the signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in said amplitude and phase (hereinafter referred to as “weight”). In addition, signals are transmitted in a directional pattern of the antenna corresponding to the weight.
One example of processings for weight computation in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula with smaller amounts of calculation than directly solving for the Wiener solution. For such recurrence formulas, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used (see, for example, Reference (1) in the following Related Art List).
Related Art List
(1) Japanese Patent Application Laid-Open No. 2002-76746.
There are cases where a radio apparatus provided with a plurality of antennas that constitute the adaptive antenna includes quadrature modulators and quadrature detectors corresponding respectively to the plurality of antennas and there are also cases where a structure is such that phase shifters for shifting the phase of a predetermined local frequency signal by π/2 are separately provided for the quadrature detector and the quadrature modulator. For instance, there is a case where a radio apparatus is comprised of an RF chip including an quadrature modulator and an RF chip including an quadrature detector in a combined manner. If the phase shifter is constituted by flip-flops and the initial values of those flip-flops are not specified as predetermined values at the time of power-on, the phase of received signals may deviate by 180 degrees from that of signals to be transmitted. Such a case corresponds, for instance, to a case when the phase of a phase shifter at a transmitting apparatus side is +90 degrees and the phase thereof at a receiving side is −90 degrees or a case when the phase of a phase shifter at a transmitting apparatus side is −90 degrees and the phase thereof at a receiving side is +90 degrees. In such cases, even if the weights generated based on the received signals are implemented into signals to be transmitted, there will not be formed the weights for transmission that reflect the values of appropriate weights, so that the transmission beams therefor cannot be controlled normally.